iOS / PNChart

Cleanup and refactoring

Objective-C coding conventions do not call for such conjunctions, AFAIK.

- Clean up some unusual whitespace, or lack thereof.
- Remove empty method definitions.

- Split long lines into multiple shorter lines.
- Rename variables and methods.
- Fix spelling.
- Remove empty method body.
- Remove superfluous comment.

Also, remove a few lines of crufty code.

Doesn't seem like triangular point styles are currently supported, so,
this commit removes cruft.

Fix spelling of line chart point style enum value. People could
otherwise search for 'circle' and not find anything, because of the
misspelling.

- Make more correct in terms of English grammar and spelling and so
  forth.
- Use commonly-used documentation comment syntax.

Fix project configurations. Update CocoaPods to 0.34.4.

clear the label's background color

Allow margin for inflexion points when no labels

Label improvements

[Fix] : fix label background color bug when before iOS7

Label customization in PNLineChart

fixed some value conversion issues

Fix wrong comment out code of yLabelSum

* For graphic with a lot of data, we can't display all xLabels. Now it's possible to display a fewLabels with a lot of value.
* Add method to specify the width xLabel.

improve code style

Fix Y labels and graph position when all Y values are the same